1. Field of the Invention
This invention relates to bone fusion devices, more specifically, it relates to devices which aid in the fusion of adjacent vertebral bodies in the spine.
2. Description of the Related Art
Fusion cages provide a space for inserting a bone graft between adjacent portions of bone. In time, the bone and bone graft grow together through or around the fusion cage to fuse the graft and the bone solidly together. Fusion cages may be used to connect any adjacent portions of bone, however one use is in the spine. Spinal fusion cages are primarily used in the lumbar spine, although they may also be used in the cervical or thoracic spine as well. Fusion cages can be inserted in the lumbar spine using an anterior, posterior, or lateral approach.
Spinal fusion cages are cylindrical, threaded, hollow metallic orthopedic implants with multiple side holes. Known fusion cages are constructed from a variety of materials including titanium alloys, porous tantalum, other metals, allograft bone, or ceramic material. These implants are generally designed to be filled with autologous bone material and then threaded into the disc space between two adjacent vertebral bodies. There they help to promote bony fusion between the two adjacent vertebral levels that prevents relative motion between the two vertebral levels affected by spinal instability due to generative changes and/or previous laminectomy, and other conditions associated with pain with vertebral motion.
Fusion cages are generally inserted by first opening the disc space between two vertebrae of the lumbar spine using a wedge or other device on a first side of the vertebrae. Next, a tapered plug is hammered in to hold the disc space open. A threaded opening is then drilled and tapped on a second side opposite the first side of the vertebrae for producing the equivalent of a xe2x80x9csplitxe2x80x9d threaded bore defined by the walls of the vertebrae above and below the bore. The threaded fusion cage is then threaded into the bore and the wedge is removed. The first side is then drilled and tapped before inserting a second threaded fusion cage. Typically, two threaded fusion cages are used at each invertebral disc level.
In accordance with a preferred embodiment, there is provided an expandable fusion cage. The cage comprises a tubular body having first and second open ends and a wall surface disposed between the first and second ends, in which the wall surface is formed by a plurality of intersecting elongate members, at least some of which intersect with one another at one or more points intermediate the first and second ends of the tubular body. The wall surface has a superior portion and an inferior portion, wherein the superior and inferior portions are opposite each other and each have an outer surface with a plurality of barbs thereon which extend away from the tubular body. The wall surface further defines a space inside the body that is filled with a natural and/or artificial material to stimulate bone growth. The tubular body has a first diameter which permits delivery of the tubular body into a space formed between two vertebral bodies and a second, expanded diameter, wherein expansion of the tubular body expands the space between the two vertebral bodies. In a preferred embodiment, the fusion cage further comprises a capsule surrounding the material to stimulate bone growth, wherein the portions of the capsule directly adjacent to the tubular body comprise a relatively impermeable thin layer of material and the portions of the capsule adjacent to the first and second open ends comprise a material permeable by the material to stimulate bone growth.